Strontium ferrites of the general formula SrFe12O19 have long been used in large quantity as magnetically hard ferrites. It is known that the magnetic properties can be influenced by adding various elements, such as, for example, cobalt and lanthanum. Thus, for example, cobalt is used for increasing the remanence BR and lanthanum for increasing the coercive force HCJ and the breaking strength. Such a modified strontium ferrite has the general formula: Sr1-xLaxFe12-yCoyO19 in which x=y=0.01-1.00 or x=0.15 and y=x/1.6n to =x/2.6n, and n is the Fe2O3:SrO ratio used and can assume a value of 5-6.
Apart from the exact amount of the respective additives, such a ferrite is greatly influenced by the preparation process, in particular by the conditions during sintering. Thus, it is generally found that an increase in the sintering temperature leads to an increase in the remanence BR and to a decrease in the coercive force HCJ. Conversely, a low sintering temperature has a positive effect on the coercive force HCJ but reduces the remanence BR. Such modified strontium ferrites are disclosed inter alia, in WO 99/34376 A1, EP 1465214 A1, US 20040151661 A1 or WO 03/056578 A1.
Moreover, EP 0964411 A1 describes a modified strontium ferrite of the abovementioned form, in which x=y=0.15 is fulfilled. This strontium ferrite has a high coercive force HCJ of 4500 Oe. No information is given regarding the remanence BR. From this publication, it is evident that small changes in the proportions of cobalt, of lanthanum or of oxygen lead to considerable changes in the magnetic properties. Moreover, the magnetic properties can be influenced by whether lanthanum and cobalt are added to the strontium ferrite before the calcination or after the calcination.
WO 98/03864 moreover discloses such a modified strontium ferrite, extensive investigations showing that the relationship between the concentration of cobalt and lanthanum and the magnetic properties is very complex. According to this publication, a modified strontium ferrite in which the proportion of cobalt and lanthanum in which x=y=0.3 is preferred.
It was therefore an object of the invention to provide a modified strontium ferrite which has improved magnetic properties, in particular which has a high remanence BR and at the same time a high coercive force HCJ, and in which the squareness of the demagnetization curve, expressed by HK/HCJ, is high. HK is the field strength at which the remanence is irreversibly reduced by 5%.